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Four months after a massive fireball destroyed a Falcon 9 rocket, its Amos 6 communications satellite payload and much of Space Launch Complex 40, SpaceX aims to return its flagship launch vehicle to flight on January 8. A painstaking investigation, conducted under the oversight of the FAA, has determined the cause of the explosion lies in the failure of one of the three liquid helium composite overwrapped pressure vessels (COPVs) coolant tanks in the second stage.

“[T]he investigation team concluded the failure was likely due to the accumulation of oxygen between the COPV liner and overwrap in a void or a buckle in the liner, leading to ignition and the subsequent failure of the COPV,” SpaceX said in a statement posted yesterday morning.

The helium tanks are placed inside the liquid oxygen tank. Essentially, the failure mode involves liquid oxygen (LOX) becoming trapped between the outer composite layer of the helium tank and the metal tank itself. Additionally, some of the LOX can freeze solid when exposed to the temperature of liquid helium. As a result, the oxygen and carbon may react under the stress of friction and ignite even in the absence of a heat or electrical source.

“The recovered COPVs showed buckles in their liners,” the SpaceX statement continued. “Although buckles were not shown to burst a COPV on their own, investigators concluded that super chilled LOX can pool in these buckles under the overwrap. When pressurized, oxygen pooled in this buckle can become trapped; in turn, breaking fibers or friction can ignite the oxygen in the overwrap, causing the COPV to fail.”

In order to return to flight as soon as practical, SpaceX identified changes to the COPV configuration and fuel loading procedures which will mitigate the presence of trapped oxygen. If the FAA signs off on the changes (the FAA is the responsible authority regulating commercial launches) and issues a launch license in the coming days, SpaceX is targeting launch next Sunday, January 8, at 10:28 am from SLC-4E at Vandenberg AFB, CA on a long-awaited mission to deliver 10 Iridium NEXT satellites to orbit.

The temporary “fix” involves loading the helium slower and at a higher temperature, essentially the procedure that the company utilized before switching to super-chilled “densified” propellant. While this will make it more difficult to return the Falcon 9 first stage to a landing complex, landings on ASDS are still feasible.

For the longer term, SpaceX says they are redeisinging the COPV’s to eliminate buckles and the possibility of LOX being trapped under the composite wrap. This will enable super-chilled propellants to be used and landings back on solid ground.

The full text of SpaceX’s statement follows:

“Over the past four months, officials at the FAA, the U.S. Air Force (USAF), the National Aeronautics and Space Administration (NASA), the National Transportation Safety Board (NTSB), along with several industry experts, have collaborated with SpaceX on a rigorous investigation to determine the cause of the anomaly that occurred September 1 at Space Launch Complex 40 (SLC-40) at Cape Canaveral Air Force Station in Florida. This investigation team was established according to SpaceX’s accident investigation plan as approved by the FAA. As the primary federal licensing body, the FAA provided oversight and coordination for the investigation.”

“Investigators scoured more than 3,000 channels of video and telemetry data covering a very brief timeline of events – there were just 93 milliseconds from the first sign of anomalous data to the loss of the second stage, followed by loss of the vehicle. Because the failure occurred on the ground, investigators were also able to review umbilical data, ground-based video, and physical debris. To validate investigation analysis and findings, SpaceX conducted a wide range of tests at its facilities in Hawthorne, California and McGregor, Texas.”

“The accident investigation team worked systematically through an extensive fault tree analysis and concluded that one of the three composite overwrapped pressure vessels (COPVs) inside the second stage liquid oxygen (LOX) tank failed. Specifically, the investigation team concluded the failure was likely due to the accumulation of oxygen between the COPV liner and overwrap in a void or a buckle in the liner, leading to ignition and the subsequent failure of the COPV.”

“Each stage of Falcon 9 uses COPVs to store cold helium which is used to maintain tank pressure, and each COPV consists of an aluminum inner liner with a carbon overwrap. The recovered COPVs showed buckles in their liners. Although buckles were not shown to burst a COPV on their own, investigators concluded that super chilled LOX can pool in these buckles under the overwrap. When pressurized, oxygen pooled in this buckle can become trapped; in turn, breaking fibers or friction can ignite the oxygen in the overwrap, causing the COPV to fail. In addition, investigators determined that the loading temperature of the helium was cold enough to create solid oxygen (SOX), which exacerbates the possibility of oxygen becoming trapped as well as the likelihood of friction ignition.”

“The investigation team identified several credible causes for the COPV failure, all of which involve accumulation of super chilled LOX or SOX in buckles under the overwrap. The corrective actions address all credible causes and focus on changes which avoid the conditions that led to these credible causes. In the short term, this entails changing the COPV configuration to allow warmer temperature helium to be loaded, as well as returning helium loading operations to a prior flight proven configuration based on operations used in over 700 successful COPV loads. In the long term, SpaceX will implement design changes to the COPVs to prevent buckles altogether, which will allow for faster loading operations.”

“SpaceX is targeting return to flight from Vandenberg’s Space Launch Complex 4E (SLC-4E) with the Iridium NEXT launch on January 8. SpaceX greatly appreciates the support of our customers and partners throughout this process, and we look forward to fulfilling our manifest in 2017 and beyond.”